1. Field of the Invention
This invention relates to magnetostrictive elements such as magnetostrictive oscillators, actuators, and sensors.
2. Prior Art
Magnetostrictive materials not only undergo deformation or strain upon application of a magnetic field, but also have the reverse magnetostriction effect, known as Hilary effect, that they develop a magnetic field when deformed. There are various proposed active and passive elements utilizing magnetostrictive properties, for example, magnetostrictive oscillators, actuators, and sensors. In particular, RFe.sub.2 Laves structure intermetallic compounds as typified by TbFe.sub.2 are known as giant-magnetostrictive materials because of their extremely increased magnetostriction. Great attention is now paid to magnetostrictive elements using giant-magnetostrictive materials.
In general, these magnetostrictive elements have the following structure and performance.
A magnetostrictive sensor includes a magnetostrictive member and a magnet which are generally arranged in series and in close contact for producing a biased magnetic field. A coil is wound around the assembly. When the magnetostrictive sensor receives external vibration, impact, or any force that deforms the magnetostrictive member, electric current is induced in the coil due to Hilary effect. By measuring the current flow or voltage across the coil, the force applied to the sensor can be determined.
A magnetostrictive oscillator or actuator includes a magnetostrictive member, a magnet and a coil arranged in the same fashion as the magnetostrictive sensor. Alternating current, pulse current or direct current is applied to the coil to produce a magnetic field. Utilizing the vibration or deformation caused by the magnetic field, the element performs as an oscillator or actuator. Magnetostrictive elements of this construction can also be used as acoustic elements.
In these magnetostrictive elements, the closed magnetic circuit can be configured as desired by inserting a magnetic material yoke in series and contact with the magnetostrictive member and the magnet.
As the applied magnetic field changes in intensity, a magnetostrictive member changes its magnetostriction quantity. However, the resultant deformation is not in proportion to the applied magnetic field. Thus, a DC biased magnetic field is often applied to the element in order that the region where maximum deformation is exerted relative to a change in the applied magnetic field intensity is utilized in the case of magnetostrictive oscillators or actuators, and in order that the deformation of the magnetostrictive member changes linearly relative to a change in the applied magnetic field intensity in the case of magnetostrictive sensors.
A DC biased magnetic field is applied by various techniques, for example, by conducting direct current across the coil or using a magnet. In the DC conduction technique, the biased magnetic field can be readily changed in intensity by changing the magnitude of the current. However, where direct current is overlappingly applied across the coil across which alternating current is applied for driving the magnetostrictive member, chokes, capacitors and other components are necessary in order to avoid any interference between the AC and DC sources. This complicates the construction and increases the size of the element. It is possible to provide a magnetostrictive member driving coil and a biased magnetic field producing coil separately although this also results in an element having complicated construction and an increased size.
On the other hand, the use of a magnet for producing a biased magnetic field ensures a compact magnetostrictive element of simple construction.
Giant-magnetostrictive materials as mentioned above have low magnetic permeability. If a magnet is used to apply a biased magnetic field to a magnetostrictive member of giant-magnetostrictive material, then the magnetostrictive member exhibits a substantially uneven magnetization distribution that magnetic susceptibility is high in regions near the magnet, but drastically lowers in remoter regions. This means that the member is magnetized to optimum intensity in some regions, but insufficiently in other regions, failing to increase the overall efficiency of the magnetostrictive member.